1. Field of the Invention
The invention relates to a half-tone printing process.
2. Description of the Relevant Art
Stencils for textile printing, which apply different quantities of colour per unit area, area by area, induced by the pattern produced (half-tone printing) are generally known. These stencils may be flat stencils or rotary printing stencils. These stencils may be screen-printing stencils, flexographic printing stencils, gravure printing stencils, etc. What is common to them all is that they have in different stencil regions stencil opening structures which are uniform but different from region to region. In the case of screen-printing stencils, in different stencil regions the screen openings of a screen which is coated with covering varnish is covered or exposed to a different extent, in order to achieve different degrees of permeability in the respective regions. However, the screen could also be manufactured from the start in such a way that it has different screen openings in different regions. The latter is also true for the flexographic stencils or gravure stencils also mentioned above, in which however, no through-openings are present. Here, the term screen opening structure refers to the depressions present in the surface of the said stencils.
As an example, let the production of a rotary screen-printing stencil be explained in more detail. Here, it is possible to use a hollow cylindrical round screen which is coated with lacquer and is thus closed and, for example by means of a laser which removes the lacquer coating, can be opened over the entire area or partially. If the lacquer layer can be polymerized, the laser could also be used only for the point by point exposure of the lacquer layer, in order to cure the latter. A development process would then be carried out in order to remove the non-exposed regions of the varnish layer. However, it is also possible to produce a pattern consisting of a large number of small, for example hexagonal, screen points of different size, on a nickel cylinder (hollow cylinder) which is coated with lacquer, by on the one hand removing the lacquer or, on the other hand, by exposing and developing the lacquer. In any case, a laser beam is caused to scan the surface of the screen or of the hollow cylinder, for example following closely adjacent helical lines, and the laser beam is pulsed. In the case of the hollow cylindrical round screen, the varnish is thus removed from the screen cylinder in the form of small openings, and a non-uniform, perforated varnish structure is then overlaid on the uniformly perforated screen. The stencil which is produced in this way can be used directly for printing. In the other case mentioned, of the completely closed cover of a hollow cylinder, a varnish structure or stencil opening structure consisting of a large number of small and separate points is produced by means of the laser beam via the abovementioned processing. These points are produced in different sizes, to be specific from pattern region to pattern region, and the cylinder formed in this way is subjected to a further electroplating process in order to coat it with nickel. In this case, nickel is deposited at the exposed points on the metallic cover of the hollow cylinder, whereas at those points at which a varnish point has remained, a hole is produced in the nickel coating. During this electroplating process, a sleeve or electroplated stencil is thus obtained having openings distributed in accordance with the pattern and of different diameters in different pattern regions, for which reason different amounts of ink pass through the respective pattern regions later during printing.
In the case of the varnished stencils, the different permeability of the stencil regions is produced by means of on times or off times of different lengths of the engraving laser beam.
Stencils of the type mentioned above can also be produced, however, by spraying a liquid onto the stencil base cylinder. In this case, the different permeability of the stencil regions is produced by means of on times or off times of different length of a spraying nozzle which is used. For example, it would be possible by this means to coat a uniformly perforated screen in an appropriate way with covering lacquer, in order immediately to obtain different regions having in each case a different degree of permeability. This is correspondingly true for the coating lacquer layer which is sprayed onto the closed surface of a supporting body for the production by electroplating of a corresponding screen. In the case of a screen, it would also be possible to spray an opaque liquid in pattern onto a polymerizable coating layer, following which large-area exposure is carried out. Following the curing of the non-covered layer regions, a development process would then be carried out.
One of the difficulties which occurs here is that many settings of operating parameters both on the engraver or laser engraver and also on the printing machine are left to the judgement and the skill of the engraver or of the printer and, as a result, unintentional, severe deviations from the intensity profile aimed at for the half-tone print are produced precisely in the region of the half-tone print. Even if a half-tone stencil has been engraved with the nominally correct opening relationships, that is to say with the nominally correct permeability relationships or degrees of permeability, some of the inks may be applied with the wrong intensity as the result of an unintentionally wrongly set printing machine or one of the printing stations.
In the case of polychromatic printing, in particular, this immediately leads to an appreciable disturbance in the colour reproduction, which simply means that those colours whose correct reproduction is based on the maintenance of exact quantity relationships of the individual components are completely wrongly reproduced in terms of colour.